Notes From The Glyphosate Dust Up.

I don’t have much to add to the recent dustup over the WHO’s International Agency for Research on Cancer (IARC) designating that glyphosate is a probable carcinogen, but I’d like to highlight some of the most useful coverage. The issue was covered ably by Grist’s Nathanael Johnson, NPR’s Dan Charles and others.

Although the patent has now expired, Roundup was developed in 1974 by Monsanto and is often used in conjunction with crops like corn and soybeans that the company genetically modified to resist it. This allows farmers to kill weeds but not their crops. Although the herbicide has many other uses, its association with G.M.O.s has caused food activists to condemn it for years. Even so, farmers, biologists, and home gardeners throughout the world use glyphosate.

The I.A.R.C. report should change none of that. The panel of seventeen scientists from around the world concluded that glyphosate could be dangerous. The organization evaluates data collected for previously published peer-reviewed studies—which is a valuable service—but it does not conduct its own research. “ ‘Probable’ means that there was enough evidence to say it is more than possible, but not enough evidence to say it is a carcinogen,” Aaron Blair, a lead researcher on the I.A.R.C.’s study, said. Blair, a scientist emeritus at the National Cancer Institute, has studied the effects of pesticides for years. “It means you ought to be a little concerned about” glyphosate, he said.

But how concerned should we be, exactly? Scores of studies have been carried out over the past forty years and they have found no connection between glyphosate and cancer. Its use has been approved by regulatory agencies, including the Environmental Protection Agency, throughout the world. Moreover, the I.A.R.C. does not include draft analyses in its assessments because, being drafts, they are subject to change. For that reason, the group omitted the conclusions of a comprehensive new study by the German Federal Institute for Risk Assessment. The organization reviewed hundreds of toxicological studies and nearly a thousand published reports. It found, based on available data, neither “carcinogenic or mutagenic properties of glyphosate nor that glyphosate is toxic to fertility, reproduction or embryonal/fetal development in laboratory animals.”

Dan Charles:

“What the IARC performs is hazard assessment,” says Aaron Blair, who chaired the group of scientists that prepared the IARC’s assessment of glyphosate. Blair is a scientist emeritus at the National Cancer Institute. Hazard assessment, he explains, is concerned with a simple question: Could a substance cause damage “in some circumstance, at some level of exposure?” How commonly such circumstances or exposures actually occur in the real world, he says, is an entirely different question, and not one that IARC tries to answer.

In other words, the IARC is saying that glyphosate probably could cause cancer in humans, but not that it probably does.

Blair says that two types of evidence convinced the committee that the glyphosate has the potential to cause cancer. First, there were laboratory studies showing that the chemical can damage DNA and chromosomes in human cells. This type of damage can lead to the emergence of cancer. Second, Blair says, some studies showed increased rates of cancerous tumors in mice and rats that were exposed to glyphosate. These were rare forms of cancer that are unlikely to occur by themselves, adding to the evidence that glyphosate caused them.

On the other hand, studies of human health records did not turn up convincing evidence of glyphosate’s cancer-causing potential. A long-running study of farm workers, for instance, did not show higher rates of cancer among those exposed to the chemical.

Rather than simply re-state what others have said on the topic, I wanted to actually take a thorough look at the evidence supporting this classification. I work with pesticides (especially glyphosate) on a regular basis, so I take this classification very seriously. If glyphosate is indeed likely to cause cancer, I am in the group of people who is most likely to be affected. As most of the reasonable write-ups have previously noted, IARC group 2A agents are problematic mostly for occupational exposure; that is, people who work with (or around) the chemical on a regular basis over a long period of time. The general public is highly unlikely to see any ill effects from any agent with this classification based on available evidence. I’m disappointed that IARC decided to announce the classification about a year before they plan to release the full monograph that details their reason for the decision. Having their list of references sure would have been useful to determine which data they’re using to come to that conclusion. So I did a literature search for studies that included glyphosate and cancer. …

… Recently, Vox presented a very nice figure that summarized why you shouldn’t put too much faith in any single study about things that cause or cure cancer. I used that as a model to create this figure, which summarizes all of the information I could find relating glyphosate exposure to cancer.

In the figure, each point represents the relative risk of developing cancer between people who had been exposed to glyphosate and those who hadn’t. To interpret the figure, any points on the left side of the blue line (less than 1) means that, on average, people who were exposed to glyphosate were less likely to get that type of cancer. Points to the right of the blue line mean that people exposed to glyphosate were more likely to get that type of cancer. There are two important things to note about this figure. First, this is an obvious over-simplification of the data. Presenting the data this way excludes the uncertainty of the relative risk estimates. When a study presents these estimates, they usually also present 95% confidence intervals. Those intervals are critical to determining whether we should put much faith in the estimate. Generally speaking, if the confidence interval spans across 1, then we would conclude that the evidence is too weak to suggest any causal link. Even so, if we have similar numbers of points to the left and right of 1, or the points are all clustered very close to 1, we can safely conclude there is little evidence of a link.

After reviewing the scientific literature, the experts classified glyphosate, which is the active ingredient in Roundup, as Group 2A: probably carcinogenic to humans. In a brief statement explaining the new designation, the scientists cited a number of research papers, such as a study on rural Colombians who were exposed to a spray of Roundup. IARC said the study demonstrated that glyphosate can cause genotoxicity, or DNA damage, and cause cellular mutations that may result in cancer.

Keith Solomon, a University of Guelph professor emeritus and a globally recognized authority on pesticides, said the conclusion is “totally wrong.” Solomon should know because he wrote the Colombian study. “They stated there was evidence of genotoxicity and they quoted one paper to support that statement,” Solomon said.

“There’s no evidence that glyphosate is genotoxic.”

Solomon and an international team of scientists conducted a study on glyphosate in Colombia in the early 2000s as part of a Colombian government program to destroy illegal coca fields in the countryside.